1. Field of the Invention
This invention relates to an improved high speed catadioptric system with a front lens, a main mirror lens with an unsilvered central portion, a counter mirror and a field lens system.
2. Description of the Prior Art
High speed catadioptric systems with spherical faces are known. These systems, with only rare exceptions, are variations of the Mandler-type and have two or more front lenses, a Cassegrain mirror system and a field lens system. Such an objective is disclosed in F. I. Havlicek, L. Canzek: "Zum Korrigieren von Spiegelobjectiven", Deutsche Geodatische Kommission bei der Bayerischen Akademie der Wissenschaften, Reihe A, Heft 42, Munchen 1963. Another embodiment with a relative aperture of 1:1.2 is shown in German DE-Patent 2 222 864. A large drawback of this lens type is the great zonal error of spheric aberration.
A high speed mirror lens with good correction of the zonal error of spherical aberration is made known by Swiss CH-Patent 542 454 and by L. Canzek: "Lichtstarkes katadioptrisches Objektiv", Optica Acta, No. 12, 1971 and No. 4, 1972. This lens consists of only one front lens with its silvered central back portion effective as counter mirror, a centrally bored main mirror lens and a field lens system. The system focus is within the objective, impeding general use.
Finally, four objective lenses of such type are presented in L. Canzek: "Neue Richtung in der Entwicklung der katadioptrischen Objektive", Optica Acta, No. 2, 1979 and described in the inventor's co-pending application Ser. No. 967,973, filed Dec. 8, 1978 now U.S. Pat. No. 4,273,425. With these systems the relative aperture is 1:1.2 but the focus is placed out of the objective. They present the state of the art most nearly connected with the present application. The first two embodiments of these four are constructed most simply. They consist of one front lens and one main mirror lens. A mirror layer evaporated on the central back portion of the front lens is effective as counter mirror, and the unsilvered central portion of the main mirror lens is left without a bore. This central portion is dioptrically effective and carries one or two field lenses on its object side.
Great disadvantages of these objectives reside in the fact that high refractive index glasses SF 3 and respectively SF 55 or LaF 23 must be used for the big optical elements, i.e. for the front lens and the main mirror lens. Moreover, the front lens and main mirror lens must be manufactured from different types of glass. The well known advantages of high refractive index glasses for the correction of aberrations are in contrast to the drawbacks, namely low resistance to chemical attack, high specific gravity and a high cost. Lens manufacture from such glass is complicated and costly. Although high refractive index glasses are available where the said drawbacks are less pronounced, these glasses have a reduced transparency for the small wavelength (short-wave) part of the visible spectrum. But this spectral range is rather important for its use in optoelectronics, photography and for visual devices.
More suitable glasses are provided for the front lens and the main mirror lens of two other embodiments from the above application. But this is brought about by a more expensive construction for these objectives have an additional medium sized lens arranged near to the counter mirror. Moreover the large sized optical elements are still made from different types of glass. The advantage of the more suitable glasses is thus counterbalanced by a more complicated and expensive construction.
There are also known optical systems with a field lens system consisting of the central portion of the main mirror lens and lenses immediately adjacent on its object and image side. The effect of the lens in front of the main mirror lens as well as that of the overall field system is divergent, whereas the lens on the image side is convergent. These objective lenses likewise are of a rather expensive construction and have a very low aperture ratio. For instance, embodiments according to U.S. Pat. No. 3,490,831 with an aperture ratio of 1:10 comprise a two component front lens and a medium sized lens at the counter mirror. On the other hand objectives according to U.S. Pat. No. 4,165,151 with an aperture ratio of 1:7 comprise only a single front lens, but an additional lens and a glass carrier for the mirror are inserted at the counter mirror, i.e. two medium size additional optical elements.